This invention relates generally to nonwoven fabrics and relates more specifically to composite nonwoven fabrics that comprise a blend of metal fibers and nonmetal fibers. This invention also relates to methods for forming such composite nonwoven fabrics.
It has long been known to use nonwoven textile fabrics for disposable diapers, fabric softener sheets, disposable medical garments, automotive trim fabric, and the like. Such nonwoven fabrics are commonly made of polymer fibers by various known processes. In general, the processes include a web forming step to organize the fibers into a web structure and a web bonding step to interconnect the fibers that comprise the web in an integrated structure.
The web forming step may entail a dry laid process, or a wet laid process. Known apparatus for dry laid processes include carding machines, garnetts and air laying machines. In commonly known wet laid processes, the fibers are suspended in a water based slurry and then caused to be laid down in a method resembling papermaking.
One method for web bonding is latex, resin, or foam bonding, in which an adhesive resin is impregnated into or sprayed onto the polymeric web to bond the fibers. Another method is thermal bonding which entails heating the surfaces of the polymeric fibers to fuse the fibers to one another. Optionally, the fibers may be laced with adhesive powder prior to fusing. A well-known mechanical bonding method is needlepunching, which uses barbed needles to punch vertically through the formed web causing the fibers to interengage and become entangled with one another. Another mechanical bonding method, known as stitchbonding, uses a continuous strand of fiber to sew a stitched pattern into a formed web.
The above-described processes and apparatus for making nonwoven fabrics are described in xe2x80x9cThe Non-Woven Fabric Handbook,xe2x80x9d by the Association of the Non-Woven Fabrics Industry. See also, Smith et al., U.S. Pat. No. 4,888,234, the contents of which are incorporated herein by reference.
Nonwoven fabrics comprised of metal fibers are also known. For example, Webber, U.S. Pat. No. Re. 28,470 discloses a nonwoven metal fabric comprising staple length metal fibers. The metal fibers are produced by bundle drawing, in a method similar to drawing wire. The metal fibers are then cut into appropriate lengths, and formed into a web. The metal web material is layered or laminated and compacted and/or annealed to form. a porous web structure.
Nonwoven metal fabrics are useful in various industrial, chemical and biological filtration processes. Another important application for nonwoven metal fabrics is as abrasive polishing pads which may be used in xe2x80x9csandingxe2x80x9d or finishing wood products, removing rust from metallic surfaces, or buffing and polishing floors.
Nonwoven metal fabrics, for example, are particularly well suited for use as buffing pads for use with electric rotary floor buffing machines. Steel wool buffing pads have been known in the art for some time, and have advantages over grit based polishing pads such as those comprising a synthetic nonwoven fabric sprayed with an abrasive coating containing a desired amount of grit. Such grit based polishing pads polish surfaces by forming tiny scratches in the surface being polished. Steel wool buffing pads on the other hand, tend only to remove surface imperfections and bumps protruding above the surface being polished without actually scratching into the surface. Therefore, steel wool buffing pads tend not to wear the surface nearly as much as grit based pads. However, while steel wool buffing pads exhibit superior polishing qualities, they tend to wear out more quickly than their synthetic grit based counterparts. In order to strengthen steel wool polishing pads, pads have been formed from needle punched steel wool fabric.
Given the shortcomings of existing nonwoven metal fabrics, it is desirable it provide an improved nonwoven fabric that combines the advantages of steel wool or other metal fibers with the advantages of nonwoven fabrics formed of synthetic or other non-metal fibers. Such an improved nonwoven fabric should advantageously provide improved isotropic strength and greater durability, so that the improved fabric will be well suited for use as an abrasive in commercial sanding machines, and floor buffing machines, as well as other applications where it is useful to combine the advantages of metal and non-metal fibers.
It has been discovered that extremely strong nonwoven fabrics may be provided that comprise layers of a composite web material of metal and nonmetal fibers formed into an integrated matrix structure. The metal fibers preferably have rough outer surfaces that are irregular in cross-section with barbed projections. The nonmetal fibers are preferably crimped synthetic fibers. The intertwined mix of metal and nonmetal fibers comprising the nonwoven fabrics of the present invention provides surprising isotropic strength and structural integrity to the fabrics, providing improved performance features not heretofore achievable in single component nonwoven fabrics.
The composite nonwoven fabrics of the present invention comprise metal fibers having an average cross-sectional diameter of from about 25 microns to 125 microns or more, and preferably have an average diameter of 50 microns or more. Fibers greater than 50 microns in diameter are stronger, and do not break as easily as smaller fibers. Thus, the use of metal fibers having an average diameter greater than about 50 microns strengthens the composite nonwoven fabrics of the present invention. The barbs and irregular surfaces of the metal fibers provide the composite non-woven fabric a desired abrasive quality, and helps maintain the interentanglement of the fibers. The abrasiveness, however, tends to be tempered by the commingling of the smoother and softer nonmetal fibers. Therefore, the strength and abrasiveness of the fabric can be controlled by careful manipulation of the mix of metal and non-metal fibers. Variables that can be controlled include the size of the fibers and the weight ratios between the metal and nonmetal fibers used in the product.
In a preferred embodiment the composite matrix fabric of the present invention forms an improved floor buffing pad. The nonmetal fibers comprise plastic strands of polyester, polypropylene or other suitable plastic material or other nonmetallic fibers, like cotton. As noted above, the composition of the composite matrix may be varied in order to maximize certain characteristics such as strength, durability or abrasiveness. The weight ratio between metal and nonmetal fibers may vary anywhere from as great as 20 parts metal fibers to one part nonmetal fibers and more, to as little as 5-parts metal fibers to one part non-metal fibers or less. In the preferred embodiment of a floor buffing pad, the preferred weight ratio between metal and nonmetal fibers is in the range between 9-10 parts metal fiber to one part non-metal fibers. Given the densities of typical metal fibers such as steel wool, and non-metal fibers such as polyester, this corresponds to a near one-to-one fiber-to-fiber ratio. Preferably, the length of the fibers will be in the range between 1-6 inches long with 3 inch fibers preferred. The cross sectional diameter of the fibers is best between 25 to 125 microns with 50 microns preferred. This mix of metal and nonmetal fibers provides a fabric having isotropic strength and abrasiveness particularly well suited for use in floor buffing. Individual circular floor pads may be stamped, or die cut from large sheets of raw composite fabric.